An effort by U.S. steel companies to block the flow of cheap foreign steel into the United States received a boost this week with a preliminary ruling by the U.S. Commerce Department that six countries were "dumping" steel pipe in the United States.

The determination came three weeks after import quotas expired and talks on a multinational steel agreement broke down in Geneva. Since then, Bethlehem Steel Corp. and Inland Steel Industries Inc. have filed unfair-trade charges against steel producers in four countries.

In its preliminary finding, issued Wednesday, the Commerce Department said steel producers in Brazil, South Korea, Mexico, Romania, Taiwan and Venezuela were selling small-diameter steel pipe in the United States at unfairly low prices. This pipe, known as circular welded non-alloy steel pipe, is used for plumbing, heating and air-conditioning systems, fencing and other construction-related purposes.

The department is expected to make a final determination on the products from Mexico, Romania, Taiwan and Venezuela July 6. A decision on products from Brazil and South Korea is expected Sept. 4.

If the department decides that the imports are hurting the U.S. steel industry, it can impose import duties ranging from 2.09 percent for one South Korean company to 96.29 percent for a company in Mexico.

The Commerce Department's finding stems from an Oct. 15 petition filed against the foreign pipe producers by the Committee on Pipe and Tube Imports, a trade association of U.S. pipe makers.

U.S.-made steel pipe has been hit hard by low-priced imports, said Roger Schagrin, a spokesman for the committee. In the early 1980s, imports accounted for as much as 60 percent of the domestic market, he said.

Even with the quotas, known as voluntary restraint agreements, imports controlled about 40 percent of the market, Mr. Schagrin said.

Bethlehem Steel, which owns the Sparrows Point mill in Baltimore County, is not directly affected by the Commerce Department's ruling, since it sold its three pipe mills at Sparrows Point in 1970 and 1983.

The company now only makes large pipes, ranging from 16 to 42 inches in diameter, at its Steelton, Pa. plant.

Bethlehem and Inland filed petitions April 13 in a separate case charging unfair-trade practices by producers of specially made steel bar and rod products in Brazil, France, Germany and Britain.

The petitions, filed with the Commerce Department and the U.S. International Trade Commission, name only makers of leaded or bismuth carbon steel bars and rods.

Those companies make specialized products that are used primarily in spark plugs and other automotive parts.